This invention relates, generally, to light measurement and, in particular, to a device for measuring flicker which may be present in an illumination source.
It is known that the human eye is sensitive to light flicker as low as one-half to one percent between 2 and 20 Hz. Large fluctuating electric loads, such as rolling mills and arc furnaces may create voltage disturbances which create such light flicker. Traditionally, methods such as synchronous capacitors or mechanically switched capacitor banks have been used to reduce these disturbances. These method suffer from the disadvantage that they are slow acting with respect to high frequency disturbances (20 Hz). With the advent of static VAR generators which are used for voltage regulation, fast disturbances may be minimized. However, the performance of static VAR generators in relation to light flicker is typically based on a percent regulation of line voltage. Since the relation between voltage and light output varies widely amongst differing light sources such as incandescent, mercury vapor, as well fluorescent lamps using different ballasts, the actual light flicker when based on percent regulation of line voltage is not readily identifiable. Traditional methods of measuring light do not allow the measurement of the actual flicker and generally do not appear on a meter since meters generally tend to damp out such small oscillations, thereby making the measurement of actual flicker inaccurate and inconsistent. One typical method of measuring lighting effectiveness may be found in U.S. Pat. No. 4,008,391, "Device For Measuring Lighting Effectiveness", issued Feb. 15, 1977 to Henderson, Jr., however even this only measures the quantity of light available.
It is desirable to have a device which measures as an identifiable quantity, the flicker present in illumination systems. It is also desirable that such a measurement exclude the average light output as well as any ripple which may be produced by the frequency of the power being supplied to the illumination system. Such a scheme is taught in the present invention.
Accordingly, the present invention relates to an illumination flicker measuring device, comprising a light sensor having an output for producing a first output signal related to the output of a measured light source, a first capacitor connected to receive the first output signal for removing the average value of the first output signal, thereby producing a second output signal, a first rectifier-amplifier device having an input connected to receive the second output signal for removing the lower envelope of the first output signal thereby producing a third output signal, a filter device having an input connected to receive the third output signal for removing that component of the first output signal ripple which is produced by the current supplied to the light source and producing a filtered output signal and a second capacitor connected to receive the filtered output signal of the filter for removing the direct current components of the first output signal thereby providing a signal which is related to the flicker produced by the light source.